The Toyota Prius has two batteries. The type of the largest one, which is the heart of the Hybrid power system, depends on the engine model and generation.
The 2G/3G/4G Toyotal Prius (I have a 2G Prius which is the XW20 engine used in the 2005–09 models), uses a NiMH (Nickel Metal Hydride Battery). The Plug-in Prius, which has a 4G engine, features a Lithium Ion battery.
The second battery, which is located in the rear-side storage compartment of the Prius, is just a regular 12-volt dry-cell car battery specially designed for the Toyota Prius. It looks like this:
The 12-volt battery is responsible for starting the engine.
SOURCE: www.quora.com